There are many occasions where the measurement of the surface potential or surface voltage, surface potential distribution, polarity or density of charge on a surface or even measurement of current density emitted by a charge source is desirable. Also, there are situations where quantification of the efficiency of electrostatic charging devices is needed. This invention deals with a probe that efficiently measures, for example, the charge density on a charged surface at a resolution level of interest to the analyst which can fall in the sub-micron range.
While the present probe can be effectively used with a plurality of different charged surfaces, for clarity and understanding, it will be described when used in an electrostatic marking system, such as electrophotography.
Electrostatography is best exemplified by the process of xerography as first described in U.S. Pat. No. 2,297,691 to C. F. Carlson. In this process, the photoconductor is first provided with a uniform electrostatic charge over its surface and is then exposed to image-wise activating electromagnetic radiation that dissipates the charge in illuminated areas of the photoconductor while the charge in the non-illuminated areas is retained thereby forming a latent electrostatic image. This latent electrostatic image is then developed or made visible by the deposition of finely-divided electroscopic marking particles referred to in the art as “toner.” The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electrostatic image. This powder image may then be transferred to a support surface, such as paper. The transferred image may subsequently be permanently affixed to the support by heat fusing. Instead of forming a latent image by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, a latent image may be formed by charging an insulating or photoconductive insulating member in image configuration by suitable process, for example by ion writing, as described in U.S. Pat. No. 5,257,045 issued to Bergen, et. al. and also known in the art as ionography. The powder image may be fixed to the imaging member if elimination of the powder image transfer step is desired. Alternatively, the toned image can be transferred onto and off from an intermediate surface, such as an intermediate transfer belt's surface, before transfer and fixing to final desired media.
Several methods are known for applying an electrostatic charge to the large area of a photosensitive member such as the use of ion generating devices such as single corona-charging structures such as metal wires, saw tooth shaped pins, insulator-coated wire assemblies, and biased charging rollers or belts. In recent development of high speed xerographic reproduction machines where copiers and/or printers can produce output at rates in excess of three thousand copies per hour, and where precise management of charge upon a plurality of surfaces is required, such as on photosensitive imaging surfaces, intermediate transfer belts and related rolls, fusing and pressure rolls and belts, as well as on most moving surfaces in an electrographic printer, the need for uniform and reliable charging and charge controlling systems are needed in order to provide optimum image quality as well as reliable printer operation. Also, with the advent of color copiers, printers, and multifunctional devices (that copy/print/fax) where several corona-charging stations are needed, the requirement for dependable means for depositing a uniform electrostatic charge is essential. With the advent and progress of contemporary digital half-toning algorithms, photoreceptor charge uniformity is more important than ever to ensure good halftone quality of the printer's output.
Generally, in electrostatographic or electrostatic copy processes, a number of corotrons or dicorotrons are used at various stations around the photoreceptor. For example, the dicorotrons are used at the station that places the initial uniform charge on the photoreceptor, at a transfer or pre-transfer station, at a cleaning or pre-cleaning station, at an erase station, etc. In today's high speed copiers where reliable and uniform charging of numerous, high speed moving surfaces is required, it is important that all corotrons (or dicorotrons) are in consistent, perfect working order since corotron malfunction or contamination can easily create streaks and non-uniformities in the output of the xerographic engines in which they are used. Some high speed engines, including color copiers and printers, use several dicorotron units, and, may include as many as sixteen corotron or dicorotron units in engines that employ image-on-image technology like the Xerox iGen3 color press. Maintaining each corotron unit in perfect working order is essential to the proper functioning of these complex, high speed color engines. It is common to use one or several corona-generating device(s) (“corotron” or “dicorotron”) for depositing the electrostatic charges at the above-noted stations. A reliable, compact, and low cost probe for quantifying the uniformity of these xerographic charging devices is needed.
Further, there are instances where it is important to measure and monitor the macro- and micro-uniformity of the output of xerographic charging devices, and other instances where measurement of such parameters as the maximum, the minimum, local variations, and/or the mean sample charge density on large smooth, or irregular, or toned surfaces may be required. Since the difference between closely spaced charges is often a determinant of the desired surface state, which is generally beyond the capability of contemporary measurement systems, there exists a need to increase the detection resolution to a level of at least the size of the typical toner particles (5 to 10 microns) and preferable in the range of about 100 nanometer (nm) resolution or even 10 nm or lower. In so doing this high resolution sensing capability would enable direct mapping of microscopic charge domains including closely-spaced, non-uniform charges such as “hot” or “cold” spots in or on a plurality of surfaces as well as directly from the charging devices themselves such as the charge emissions from nano-structured charging devices such as the ones described in US Patent Application Publication 20060280524, filed on Dec. 14, 2006 by Hays, et. al. In addition, there is a need to make the devices more efficient and reliable, for example, to enable simultaneous maps of more than one microscopic area within a large field on a photoconductive or other surface of interest that must operate in the open environment. Similarly, there is a need to make the sensing devices small or compact in size, low in cost, safe to use, and easy to manufacture.